In, for example, MF broadcasting, a transmitter uses a digital amplitude modulation device. The digital amplitude modulation device includes a plurality of power amplifiers arranged in parallel. The digital amplitude modulation device ON/OFF-controls the plurality of power amplifiers based on the voltage amplitude level of a modulated signal such as an audio signal supplied from the outside, thereby amplifying a carrier signal while changing the number of power amplifiers to be driven. The digital amplitude modulation device composites the output signals of the power amplifiers in the ON state to generate an AM wave (Amplitude Modulation Wave), and sends out the generated AM wave to a predetermined broadcasting service area.
In a digital amplitude modulation device of this type, if some kind of fault such as breakage of a power amplifier or short-circuit/open-circuit of the compositor occurs, the signal quality of the AM wave may degrade. It is therefore necessary to prevent the power amplifiers from breaking at the time of, for example, a lightning strike, load system abnormalities such as disconnection, malfunction, and surging, and degradation in SWR (Standing Wave Ratio) caused by the load characteristic of an antenna.
To prevent breakage of power amplifiers, the digital amplitude modulation device measures the voltage and the current in the process of transmitting the AM wave and controls units in the devices based on the measurement result. For example, a method of reducing output power to 0 by turning off the switch of the transmission circuit and a method of reducing transmission power to the power amplifiers when an SWR value calculated based on the measured voltage and current exceeds a warning value limit at which the power amplifiers may break are known.
To calculate the SWR value, a CPU (Central Processing Unit) is used in general. Because of a large calculation delay amount, the CPU cannot follow an abrupt change in the SWR value in real time upon a lightning strike or the like. Hence, it is difficult for the digital amplitude modulation device to prevent breakage of the power amplifiers. The problem of calculation delay amount also arises even when a digital circuit formed from a CPLD (Complex Programmable Logic Device) or FPGA (Field Programmable Gate Array) is used for control to prevent breakage of the power amplifiers.